The polyphenylene ether resins are well known in the art as a class of thermoplastics which possess a number of outstanding physical properties. They can be prepared, in general, by oxidative and non-oxidative methods, such as are disclosed, for example, in Hay, U.S. Pat. Nos. 3,306,874 and 3,306,875 and Stamatoff, U.S. Pat. Nos. 3,257,357 and 3,257,358, which are incorporated herein by reference.
It is known that when the polyphenylene ether resins are combined with styrene resins, such as crystal grade homopolystyrene or high impact rubber modified polystyrene, there are obtained compositions having many properties which are improved over those of either the polyphenylene ether or polystyrene alone. Moreover, these respective polymers are combinable in virtually all proportions, e.g., from 1 to 99 parts of polyphenylene ether to 99 to 1 parts of polystyrene. Examples of polyphenylene ether-polystyrene compositions are disclosed in Cizek, U.S. Pat. No. 3,383,435, which is incorporated herein by reference.
In recent years, there has been increasing concern about the performance and safety of thermoplastic materials, including the afore-mentioned polyphenylene compositions during real-life fire situation. One of the potential hazards presented by the presence of thermoplastics in fires is that they can contribute to fire spread by dripping flaming and/or molten resin.
The term "dripping" has an art-recognized meaning. More particularly, in a fire situation, a thermoplastic can become hot enough to exhibit some degree of melt flow. When this flow is extensive enough to result in the physical separation of the molten mass from the main body of the plastic, "dripping" is said to have taken place. Because the dripped material is sometimes flaming and since dripped flaming resin is a means by which fire can spread to combustible surroundings, dripping is undesirable in a thermoplastic material.
Of particular interest herein are plasticized thermoplastic compositions comprised of a polyphenylene ether resin, plasticizer(s) present in plasticizing amounts, and optionally, impact modifiers such as rubber-modified, high-impact polystyrene resins or A-B-A.sup.1 block elastomeric copolymers. These compositions have now been investigated under test conditions which are even more closely related to real-life large-scale fire conditions than are more conventional and more widely used tests such as the Underwriters Laboratories Bulletin No. 94 critical burning test.
The UL-94 test is generally carried out by preparing a molded test piece of about 5".times.1/2".times.1/16", supporting the sample vertically, and igniting it. If the sample does not form flowing droplets sufficient to ignite a piece of cotton held 12 inches beneath and extinguishes itself within an average of 5 seconds after each of two 10-second ignitions, the composition is deemed to be non-dripping and flame-retardant to the point where it satisfies the V-0 requirements of the Underwriters' Laboratories. If the test sample extinguishes itself within 30 seconds, after two 10-second ignitions, the composition is deemed to be flame-retardant and non-dripping in satisfaction of the V-1 requirements.
Because the heat flux generated during the usual Underwriters Laboratories Bulletin No. 94 test is relatively small in comparison with heat flow which prevails during real, large-scale fires, the following test has now been devised which more closely simulates reality than the UL-94 test:
The barrel of a Bunsen burner is screwed down (clockwise) so that the air ports are closed. The gas flow is adjusted to produce a blue/yellow flame of approximately 5-6 inches in height. A 5".times.1/2".times.1/16" test sample is suspended vertically in the center of the flame and about 3/8" above the burner top, until either dripping takes place or 5 minutes of continuous ignition time has elapsed. PA0 (a) a polyphenylene ether resin; PA0 (b) a plasticizer in an amount at least sufficient to provide a plasticized composition after molding; and PA0 (c) a microfibrillar poly(tetrafluoroethylene) resin in an amount at least sufficient to render said thermoplastic composition non-dripping when molten and/or burning.
The above procedure is hereinafter referred to as the "flame bath" test, because the test specimen is literally immersed in flames during the test.
It has now been surprisingly discovered that the "dripping" (as measured by both the 1/16" UL-94 and "flame bath" tests) of plasticized compositions of a polyphenylene ether resin and a plasticizer can be greatly retarded or completely prevented by the incorporation of microfibrillar poly(tetrafluoroethylene), in relatively small amounts. It has been previously proposed, in co-pending U.S. application Ser. No. 530,373, filed Dec. 6, 1974, assigned to the same assignee as herein, that poly(tetrafluoroethylene) acts as an effective drip-retarding agent for self-extinguishing polyphenylene ether compositions which contain flame retarding agents. However, it has not been previously recognized that a particular kind of poly(tetrafluoroethylene), namely microfibrillar poly(tetrafluoroethylene), is a remarkably effective drip-retarding agent in plasticized polyphenylene ether compositions, even in the absence of flame retarding additives.